Screws and bolts of various designs are known and are commonly used as fasteners for assembly of useful articles. Characteristically screws have a threaded shank portion, a tip design depending on the application, and a head having one or more slots. Driver bits of various designs are configured to fit the drive slot or slots on the screw head and engage the screw by transmitting a rotary force or torque through the driver bit to the screw head, generally through the screw head slot sidewalls.
Screwdriver tips generally have a relatively short blade tip length at the portion that engages the screw head. Screws are inserted or removed from a work piece by the application of a twisting force or torque to the head of the screw. By way of definition, torque is a force applied at a distance from an axis of revolution resulting in a twisting force about the axis. In the case of a common screw the axis of revolution is through the center of the head, and the driver to head contact distance is the smaller of half the slot length or half the drive tip blade length. As is well known, the force applied by the screw driver blade tip to the slot wall of the screw head increases as the driver head to screw head contact distance decreases.
As screw heads are of rather small diameter, a given value of torque applied to drive or remove a screw from a work piece results necessarily in significant forces applied by the driver tip against the walls of the slot of the screw. This large sidewall force can result in the distortion of the slot in the screw head, requiring increasing amounts of downward force to be applied on the screw driver and tip to keep the driver tip within the screw slot and prevent it from riding out of the slot in the now deformed and damaged screw slot walls. A failure of the user to provide sufficient downward force results in the driver tip riding out of the slot and further damaging the screw sidewall and thereby requiring even more downward force to be applied. This series of events progressively damages the screw slot to the point where sufficient torque can no longer be transmitted to the screw by the driver tip as the driver tip now easily rides over and out of the now deformed slot sidewalls. This undesirable tendency of the screw head sidewalls to deform and damage is referred to herein as ‘stripping’, and is an undesirable limitation of commonly known screw and screwdriver tip designs.
In the case of self-tapping screws, these screws require that substantial amount of additional torque be applied to the screw head to achieve tapping and form of the screw hole in the piece to which it is being threaded. The additional torque is required to overcome frictional forces between the screw and the material receiving the screw, and to provide material deformation forces to enlarge and thread the hole in the work piece. For illustration with an example most readers will relate to, one common example of self-tapping screw applications are in sheet metal screws as used for instance in residential furnace heating ductwork installation. Another example would be self-tapping screws applied into hardened resin materials, metals such as steel and aluminum, and even formed aggregates such as concrete.
Therefore, a screwdriver tip and screw head design in combination that can prevent damage to the screw head slot, a screw head design that can capture the driver tip under a ledge in the head and thereby prevents the screw head from stripping, as well as reduces the amount of downward force that need be applied through the screw driver to the screw head to keep the driver tip from riding out of the screw slot, such a screw head and screwdriver tip design would be useful and novel.